(1) Field of the Invention
The present invention relates to a method of performing a fail safe control for controlled means, i.e. an engine and a fail safe control unit thereof in which different parameters representative of operating conditions of the engine are detected by detecting means and it controls and operates the engine on a safe side when any detecting means for detecting the parameters becomes defective.
(2) Description of the Prior Art
As a control unit for detecting different parameters representative of operating conditions of controlled means and for operating the controlled means as a desired condition in accordance with the parameters thus detected, there is an engine control unit having a variable capacity turbocharger. In the variable capacity turbocharger, the turbo engine is rotated by utilizing the exhaust gas of the engine and the supercharge pressure of inlet air to be supplied to the engine is controlled by the operation of a compressor by the rotation of the turbine, while the cross sectional area of inlet path or guide path of the exhaust air to the turbine is varied and the supercharger pressure of the inlet air to be supplied to the engine is properly controlled by supplying the exhaust gas from the engine to the turbine through the guide path thus controlled, thereby increasing the engine torque from a low speed zone to a high speed zone.
The Japanese Lying-Open Patent specification sho 58-162918 discloses one of these units. FIG. 1 shows a block diagram of a control unit for the variable capacity turbocharger according to the prior art.
In FIG. 1, the control unit 2 comprises an operational unit 4, a duty value calculating unit 6, an subtracter 14, and an operational unit 18. In addition, in FIG. 1, the reference numeral 1 indicates an engine, numeral 10 indicates a magnetic valve, and nemeral 12 indicates a supercharge pressure sensor 12.
The operational unit 4 calculates fuel supply pulse width T.sub.P which represents an engine load and corresponds to air flow rate for every one revolution of the engine, from the inlet air flow rate Q.sub.A and the engine speed Ne detected by an air flow meter and a crank angle sensor not indicated, respectively and it supplies the fuel supply pulse width T.sub.P and the engine speed Ne thus calcuated, to the duty value calculating unit 6.
The duty value calculating unit 6 having a stored table of suitable duty values to the fuel supply pulse width T.sub.P and the engine speed Ne, looks up the table in accordance with the fuel supply pulse width T.sub.P and the engine speed Ne inputted and produces a corresponding duty value. The duty value thus produced is applied to the electromagnetic value 10 after correction of the value through the adder, which will be described later, and is controlled in such a manner that the supercharge pressure of the inlet air to be supplied to the engine 1 through the variable capacity turbocharge becomes equal to a preset value corresponding to the duty value thus calculated.
However, also in such a control system as described above, in order to remove the effects of dispersion of the parts constituting the electromagnetic valves, actuators etc., and of the change in the time elapsed thereof, the supercharge pressure P.sub.2 of the inlet air to be supplied to the engine 1 is detected by the supercharge pressure sensor 12 and is applied to the inverting input terminal of the subtracter 14 in the control unit 2. In the non-inverting input terminal of the subtracter 14, there is supplied a target supercharge pressure P.sub.sl from the target supercharge pressure setting portion 16. In the subtracter 14, the actual supercharge pressure P.sub.2 detected by the supercharge pressure sensor 12 is subtracted from the target supercharge pressure P.sub.sl, the deviation P of the actual supercharge pressure P.sub.2 from the target supercharge presure P.sub.sl is calculated, and it is supplied to the operational unit 18. Various mathematical operations such as proportional, integral, and differentiating operations (PID operations) are carried out about the devaition P and the deviation of the duty values is calculated in the operational unit 18 and then, the result of the calculation is applied to the adder 8, thus correcting the duty value supplied from the duty value calculating unit 6.
In the control unit, the duty value as a basic control value is calculated in accordance with the parameters such as the engine speed Ne and the inlet air flow rate Q.sub.A detected by the crank angle sensor and the air flow meter and the electromagnetic valve 10 is controlled by the duty value, thus controlling the supercharge pressure of the inlet air which is supplied to the engine, so as to reach a predetermined preset value. However, when any one of sensors such as the crank angle sensor, the air flow meter, becomes defective, the values thus detected can not be obtained and a suitable control can no longer be carraied out. As a result, there is a possibility that the engine will be damaged because of an abnormally high supercharge pressure, as the case may be.
Namely, as in the case of the variable capacity turbocharger described above, in the control unit which controls the operation of the controlled means in accordance with the result of the detection of the parameters, when any detecting means for detecting the parameters becomes defective, the control can no longer become possible. Even if such a condition described above occurs, it is desirable to secure the control of the controlled means.